Learn or remember: how to choose?

We don’t know anything until we learn – logically: to master a memory, it must first be stored in the brain and, to recall it, on the contrary, it must be brought out again! How does it work? A central region of the brain, the hippocampus, is considered almost the center of memory, because it is essential not only in the early stages of its formation – this is the learning stage – but also when recalling memories. – that’s right. called the “remembering” phase. But how does the hippocampus go from memorizing to remembering? A question that has troubled researchers for a long time and which has finally been answered by Ruy Gómez-Ocádiz and his colleagues, from the Institut Pasteur and the École Normale Supérieure in Paris: it exists as a neuronal switch in the hippocampus, which makes it is possible to pass from one state, memorizing, to another, remembering.

Indeed, scientists have long studied the different ways of functioning of the hippocampus, which includes several layers of different neurons, grouped and organized in different ways depending on the layer. Thus, in the face of something new, an event or an object for example, the neurons located in the so-called “CA3” layer of the hippocampus work together to encode the information and store it in memory; then, if necessary, to understand a situation, the same neural circuit is activated to retrieve this stored data. However, these two modes of memory cannot operate at the same time: they conflict. However, in general, without realizing it, we almost constantly use new memories and information to remember to make sense of the world around us. Therefore, the two modes of the hippocampus must alternate, and it is this neural switching that Ruy Gómez-Ocádiz and his colleagues sought to find in our brains.

But to determine how the hippocampus switches from one state to another, you must succeed in confronting it with novelty almost immediately while it uses memories… A trick that physical reality does not allow! Thus, the researchers had the idea of ​​using virtual reality: they developed a video game in which rats explored and memorized virtual worlds. In parallel, they recorded the electrical activity of their hippocampus using implanted electrodes. Then they – as if by magic – teleported the rodents into an entirely new virtual “environment” while using their episodic memories of stored virtual worlds… At that moment, a weak, transient signal of electrical depolarization appeared in individual neurons : granulosa cells. of the hippocampal layer called the dentate gyrus, an area just upstream of the CA3 layer of the hippocampus. The scientists then integrated this synaptic signal of novelty, which occurs at each transition into a new virtual world, into a mathematical model of the hippocampus: it actually makes it possible to switch from remembering mode to memorizing mode and explains – finally – how we are able to use our memories as we learn new information.

A failure of this neuronal switch, that is, of switching between modes of recollection and memorization, would explain the impression of deja vu, the strange sensation of remembering something when it is first discovered? In this case, the switch of the hippocampus would be in the wrong position: he would consider that what he sees for the first time is in reality already archived in his memories. A possibility that researchers will now be able to explore.

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